Hippocampus abdominalis is the largest known species of seahorse, primarily distributed in the seas of Australia and New Zealand. It had faster growth rates, higher survival rates, stronger stress resistance, and easier breeding, thus introducing to China in 2016. Current research primarily focused on reproductive physiology, pharmacological effects and aquaculture ecology, while there had been no reports on genetic breeding research. Under high-density single-species breeding conditions, many breeding varieties would face the fate of germplasm degradation and restricted industrial development space unless selection of superior varieties was carried out promptly. Accordingly, this article launched the research on selecting superior varieties of H. abdominalis. We sought to identify the main phenotypic morphological traits that affect weight to explore the growth characteristics of H. abdominalis and analyze the impact of phenotypic morphological traits on weight at different ages. We randomly selected 60 individuals from 1 to 5-month-old H. abdominalis pools each month for four consecutive months, measured their weight and seven phenotypic traits, used correlation analysis, multiple regression analysis, and path analysis to analyze the relationships between traits, and used stepwise regression analysis to establish weight models for H. abdominalis of different ages. The results showed that the correlation coefficients between the measured phenotypic traits and weight at 1, 2, 3, and 4 months of age were highly significant (P<0.01), while at 5 months of age, except for the snout length, which was not correlated with weight, the other phenotypic traits were significantly correlated with weight (P<0.01). The phenotypic traits that most directly affected weight at 1 and 2 months of age were the length from the posterior margin of the cephalic branchial covered to the tail tip (r₁=0.615, r₂=0.334), with the highest degree of direct determination of weight (r₁²=37.8%, r₂²=11.2%). At 3, 4, and 5 months of age, the phenotypic traits that most directly affected weight were the width at the dorsal fin (r₃=0.792, r₄=0.619, r₅=0.397), with the highest degree of direct determination of weight (r₃²=62.7%, r₄²=38.3%, r₅²=15.8%). The regression coefficients R₁²=0.932, R₂²=0.915, R₃²=0.958, R₄²=0.932, R₅²= 0.877 for the selected phenotypic traits at different ages indicated that these selected traits were the main traits affecting weight. Using the dorsal fin width (DFW), cephalic gill covered long from posterior margin to caudal tip (CPCT), dorsal fin thickness (DFT), dorsal fin long from posterior margin to caudal tip (DFPCT), head and neck thickness (HNT), head length (HL) and snout length (SL) as independent variables, we established a regression equation to estimate the weight of H. abdominalis at 1-5 months of age. This research showed that in selecting H. abdominalis by weight, their growth characteristics should be fully considered, and the traits selected at different ages should be based on different criteria. This research could provide theoretical guidance for the weight selection of H. abdominalis, thereby improving selection accuracy and efficiency.